Environmental Evaluation of Different Treatment Processes for Sludge from Urban Wastewater Treatments: Anaerobic Digestion versus Thermal Processes (10 pp)

Background, Aims and Scope Huge amounts of sewage sludge, that need to be handled, are generated all around the world from wastewater treatment plants and its management in an economically and environmentally acceptable way has become a matter of increasing importance during the last few years. In this paper, we make use of Life Cycle Assessment (LCA) to compare biological and thermal processes, that is to say, anaerobic digestion versus pyrolysis and incineration. This paper will complete the analysis performed in a wastewater treatment plant, where sludge post-treatment was identified as one of the main contributors to the environmental impact on the global system. Methods LCA is a tool for evaluating the environmental performance of goods as well as processes or services (collectively termed products). ISO 14040 defines LCA as a compilation and evaluation of the inputs, outputs and the potential environmental impacts of a system throughout its life cycle: from the production of raw materials to the disposal of the waste generated. In this study, data relating to the actual scenario from an existent wastewater treatment plant were considered. Both bibliographical and real data from existing facilities were used for the thermal processes proposed. The Centre of Environmental Science (CML) of Leiden University's methodology was chosen to quantify the potential environmental impacts associated with the different scenarios under study. The software SimaPro 5.1 was used and CML factors (updated in 2002) were chosen for characterisation and normalisation stages. Results and Discussion In a previous study, sewage sludge was found to be a critical point in the environmental performance of a wastewater treatment plant, so different alternatives have been tackled here. Anaerobic digestion followed by land application of pasty sludge comprises both energy recovery and nutrient recovery. Other thermal processes, such as incineration or pyrolysis, allow energy recovery (both electrical and thermal) and, although nutrients are lost, new co-products are produced (tar and char at pyrolysis). Here, the most adverse case (that is to say, the total amount of heavy metals is supposed to be released from the sludge and reach the environment) was applied to consider the most negative impact due to sludge spreading in agricultural soils; so more research is required in order to establish the precise amount of heavy metals that is effectively uptaken by the plants and crops as well as the amount that is transferred to another phase as a leachate. Thermal processes are presented here as a good option to recover energy from the sludge; although the value of nutrients is lost. Tar and char, co-products from pyrolysis, are good examples that were evaluated here, recycling of bottom ashes from sludge incineration or manufacture of ceramic materials from sludge are other options to be studied in the near future. Conclusion During the last few years, several opinions have been declared in favour of land application, incineration or pyrolysis, but many voices have also spoken out against each one. To obtain general conclusions for an overall comparison of different post-treatment of urban wastewater sludge is not easy as there are many contradictory aspects. The most effective utilisation of sewage sludge implies both energy and material re-use, but this is not always possible. Nevertheless, we think that land application of digested sludge is an acceptable option, probably not the best but at least a good one, for sludge treatment as long as efforts are focused on the minimisation of heavy metal content in the final cake.     

污水生物处理中的噬菌体及其功能研究进展

污水生物处理是一种利用微生物分解污水中的污染物、实现污水净化的方法。噬菌体是侵染细菌的病毒,在污水生物处理系统中广泛存在,它们能够特异性地控制微生物菌群,影响污水处理效果和调控污泥性状。因此,研究污水生物处理中噬菌体的分布及其功能具有重要意义。本文介绍了不同污水生物处理中噬菌体的分布,简要分析了噬菌体分离、培养与鉴定方法及其优缺点,详细总结了噬菌体在污水生物处理中的功能,包括:(1)调节微生物群落结构,影响污水处理效果;(2)作为环境监测的指示生物;(3)控制病原菌、污泥膨胀、污泥发泡和膜污染;(4)减少污泥产量,重点分析了影响噬菌体功能的因素,探讨了污水生物处理中噬菌体功能应用存在的问题及其解决方法,最后对噬菌体未来应用的发展方向进行了展望,以期为污水生物处理技术和工艺的开发与应用提供参考,促进污水处理健康发展。     

Ferric reduction in organic matter oxidation and its applicability for anaerobic wastewater treatment: a review and future aspects

Anaerobic treatment processes have the advantages of cost-effectiveness, energy efficiency, low sludge yield and potential of resource recovery over conventional aerobic treatment methods and have been gaining increasing attention as an approach for future wastewater management. An important feature of anaerobic processes is the use of alternative electron acceptors to oxygen, which renders treatment flexibility in using redox active elements such as iron and sulfate from other waste materials. Co-treatment of acid mine drainage and municipal wastewater, as an example, has been shown to be an effective method for removing organic materials, metals, and phosphate from the both wastes. It also suggested the applicability of ferric reduction process in wastewater treatment. Most of the previous studies on ferric reduction process and iron reducers were conducted in natural systems such as sediments, soils and groundwater. This paper reviews the significance and fundamentals of the ferric reduction process, its utility for organics oxidation, controlling factors, reaction kinetics, microbial processes of iron reduction and its ecology. The paper also evaluates the suitability and discusses future aspects of using iron reduction for wastewater treatment. Knowledge gaps are identified in this paper for developing such innovative wastewater technology and process optimization.     

活性污泥中电活性微生物的富集筛选研究进展

活性污泥含有多种微生物,是废水处理系统中污染物降解转化的重要驱动者。电活性微生物(Electroactive microorganisms,EAMs)是活性污泥的重要微生物类群,在废水资源化与能源化中发挥着核心关键作用。本文概括了活性污泥中EAMs的富集和筛选方法,整理总结了目前已从活性污泥中获得的EAMs菌种资源信息,在此基础上分析了从活性污泥中富集和筛选EAMs所存在的问题,并进一步展望了未来的研究方向,以期加速EAMs菌种资源的挖掘,促进以EAMs为主导的废水资源化与能源化技术发展。     

Development of anaerobic sludge bed (ASB) reactor technologies for domestic wastewater treatment: motives and perspectives

During the treatment of raw domestic wastewater in the upflow anaerobic sludge blanket (UASB) reactor, the suspended solids (SS) present in the wastewater tend to influence negatively the methanogenic activity and the chemical oxygen demand (COD) conversion efficiency. These problems led to the emergence of various anaerobic sludge bed systems such as the expanded granular sludge bed (EGSB), the upflow anaerobic sludge blanket (UASB)-septic tank, the hydrolysis upflow sludge bed (HUSB), the two-stage reactor and the anaerobic hybrid (AH) reactor. However, these systems have, like the UASB reactor, limited performance with regard to complete treatment (e.g., removal of pathogens). In this respect, a new integrated approach for the anaerobic treatment of domestic wastewater is suggested. This approach combines a UASB reactor and a conventional completely stirred tank reactor (CSTR) for the treatment of the wastewater low in SS and sedimented primary sludge, respectively. The principal advantages of the proposed system are energy recovery from organic waste in an environmentally friendly way; lowering the negative effect of suspended solids in the UASB reactor; production of a high quality effluent for irrigation; and prevention of odour problems.     

Containment in industrial biotechnology within wastewater treatment plants

Both physical and biological containment are considered to be essential parts in the risk analysis of industrial Good Industrial Large-Scale Practice (GILSP) processes using genetically modified organisms (GMOs). Biological containment of industrial microorganisms has become a more important issue since the introduction of recombinant DNA techniques. In the event of an accidental discharge in the production plant, a large amount of organisms could be released into the wastewater treatment (WWT) system. This WWT system should therefore be considered as a part of the containment. This study demonstrates both a hydrodynamic and a microbiological model for the containment aspects of industrial WWT plants. The models are verified by measurements using industrial hosts of GILSP GMOs at full scale. Both models describe the full-scale equipment accurately. The results are supplemented with microcosm studies on survival of GMOs in defined niches. It is shown that WWT plants can be considered as useful additional parts of the containment of microorganisms, in case of an accidental discharge. The effect of drainage of an enormous amount of microorganisms (several tons) through the WWT plant into the environment is shown to be comparable to the direct drainage of a small-scale fermenter. Microcosm experiments correlate well with the survival rates in the WWT and therefore can be of use to predict the behaviour of GMOs in this environment. Journal of Industrial Microbiology & Biotechnology (2002) 28, 65–69 DOI: 10.1038/sj/jim/7000210 Received 16 July 2001/ Accepted in revised form 05 September 2001     

ROUTES: innovative solutions for municipal sludge treatment and management

Management of sewage sludge is going to be a big challenge in near future due to rapid urbanization and economic growth, in particular in Europe. The Routes project (ref. # 265156) is addressed to discover new routes in wastewater and sludge treatment which allow: (a) to prepare sludge for agricultural utilization by transforming it in a very clean and stabilized product with respect to hygienic aspects and phytotoxicity; (b) to minimize sludge production by new solutions including the use of innovative processes based on metabolic uncoupling or of innovative reactors like microbial fuel cells or sequencing batch biofilter granular reactor; (c) to promote recovery of valuable materials from anaerobic digestion, i.e. biopolymers as polyhydroxyalkanoates and fertilizers; (d) to set up and prove at practical scale a novel technique for sludge disposal (wet oxidation) as sustainable alternative to the nowadays the most used incineration; (e) to minimize energy pumping by adjusting solid concentration, on a practical installation. The general objective of the Routes proposal is therefore to set up a portfolio of different solutions to be applied in different conditions and circumstances, strictly following the waste hierarchy of the EU Directive 08/98 on waste. The above solutions will be studied either in the laboratory or at practical scale, depending on the maturity of the technology, in order to provide the Commission, the technical and scientific community and end-users with applicable solutions and new routes for sludge management.     

Microalgal-bacterial aggregates: Applications and perspectives for wastewater treatment

Research on wastewater treatment by means of microalgal-bacterial processes has become a hot topic worldwide during the last two decades. Owing to the lower energy demand for oxygenation, the enhanced nutrient removal and the potential for resource recovery, microalgal-based technologies are nowadays considered as a good alternative to conventional activated sludge treatments in many instances. Nevertheless, biomass harvesting still constitutes one of the major challenges of microalgal-bacterial systems for wastewater treatment, which is hindered by the poor settleability of microalgal biomass. In this review, the use of microalgal-bacterial aggregates (MABAs) to overcome harvesting issues and to enhance resource recovery is presented. The fundamentals of MABAs-based technologies, the operational strategies and factors affecting the formation of MABAs, the microbiology and the methanogenic potential of the aggregates are addressed and critically discussed. The most recent findings and the challenges facing this technology towards its consolidation are also presented.     

Nutrient removal and biomass production: advances in microalgal biotechnology for wastewater treatment

Owing to certain drawbacks, such as energy-intensive operations in conventional modes of wastewater treatment (WWT), there has been an extensive search for alternative strategies in treatment technology. Biological modes for treating wastewaters are one of the finest technologies in terms of economy and efficiency. An integrated biological approach with chemical flocculation is being conventionally practiced in several-sewage and effluent treatment plants around the world. Overwhelming responsiveness to treat wastewaters especially by using microalgae is due to their simplest photosynthetic mechanism and ease of acclimation to various habitats. Microalgal technology, also known as phycoremediation, has been in use for WWT since 1950s. Various strategies for the cultivation of microalgae in WWT systems are evolving faster. However, the availability of innovative approaches for maximizing the treatment efficiency, coupled with biomass productivity, remains the major bottleneck for commercialization of microalgal technology. Investment costs and invasive parameters also delimit the use of microalgae in WWT. This review critically discusses the merits and demerits of microalgal cultivation strategies recently developed for maximum pollutant removal as well as biomass productivity. Also, the potential of algal biofilm technology in pollutant removal, and harvesting the microalgal biomass using different techniques have been highlighted. Finally, an economic assessment of the currently available methods has been made to validate microalgal cultivation in wastewater at the commercial level.     

Sample preparation,preservation, and storage for volatile fatty acid quantification in biogas plants

Volatile fatty acids (VFA) represent short‐chain fatty acids consisting of six or fewer carbon atoms that can be distilled at atmospheric pressure. In anaerobic digestion processes VFAs are of central importance for maintaining stable reactor performance and biogas production, are used as indicators for arising problems and are important process monitoring parameters. In the present study, sludge derived form a full‐scale anaerobic digester of a wastewater treatment plant was spiked with formate, acetate, propionate, and butyrate in order to evaluate various commonly used techniques for VFA extraction, preservation, and storage. It was shown that VFA extraction after centrifugation warranted the highest recovery rates for spiked VFAs. Moreover, experiments clearly indicated the importance of a fast sample handling, including the necessity of immediate cooling of the samples. Chemical sample preservation within a narrow time frame or deep freezing emerged as an alternative to instant VFA extraction. Short‐time storage of extracted VFA samples at + 4°C is an option for up to 7 days, for longer periods storage at –20°C was found to be applicable.     

Changes in bacterial communities from anaerobic digesters during petroleum hydrocarbon degradation

Anaerobic biodegradation of petroleum hydrocarbons (PHC) to methane has been recognized to occur in oil reservoirs and contaminated surface sites alike. This process could be employed efficiently for the treatment of contaminated materials, including petrochemical wastes and PHC-contaminated soil, since no external electron acceptor is required. Moreover, the controlled production of methane in digestion plants, similarly to the anaerobic digestion (AD) of energy crops or organic residues, would enable for energy recovery from these wastes. At present, little is known about the bacterial communities involved in and responsible for hydrocarbon fermentation, the initial step in PHC conversion to methane. In the present study, the fate of two different methanogenic communities derived from the AD of wastewater (WWT) and of biowaste, mixed with PHC-contaminated soil (SWT), was monitored during incubation with PHC using denaturing gradient gel electrophoresis (DGGE) of 16S rDNA genes amplified with Bacteria-specific primers. During 11 months of incubation, slight but significant degradation of PHC occurred in both sludges and distinct bacterial communities were developing. In both sludges, Bacteroidetes were found. In addition, in WWT, the bacterial community was found to be dominated by Synergistetes and Proteobacteria, while Firmicutes and unidentified members were abundant in SWT. These results indicate that bacterial communities from anaerobic digesters can adapt to and degrade petroleum hydrocarbons. The decontamination of PHC-containing waste via fermentative treatment appears possible.     

Lysis and “cryptic” growth in wastewater and sludge treatment processes

The primary objectives in biological wastewater and sludge treatment processes are to procedure a minimum quantity of solid, stable residues and a maximum quantity of carbon dioxide from the organisms present in the process feeds. As far as minimizing solids production in activated sludge type processes is concerned, endogenous activity in the recycled biomass is usually considered to be the most important mechanism. However, increased understanding of the growth characteristics of mixed microbial cultures suggests that lysis and “cryptic” growth are probably dominant mechanisms. For pathogen destruction in treatment processes, death and subsequent lysis of pathogens are clearly events that must be promoted. Here, the kinetics for death, lysis and “cryptic” growth in aerobic wastewater treatment and aerobic sludge stabilization processes are examined.     

Developments in pre-treatment methods to improve anaerobic digestion of sewage sludge

During wastewater treatment, most organic matter is transferred to a solid phase commonly known as sludge or biosolids. The high cost of sludge management and the growing interest in alternative energy sources have prompted proposals for different strategies to optimize biogas production during anaerobic sludge treatment. Because of the high solid content and complex structure of sludge-derived organic matter, methane production during digestion is limited at the hydrolysis step. Therefore, large digester volume and long retention times of over 20 days are necessary to achieve adequate stabilization. Pre-treatments can be used to hydrolyze sludge and consequently improve biogas production, solids removal and sludge quality after digestion. This paper reviews the main pre-treatment processes, with emphasis on the most recent developments. An overview of the different technologies is presented, discussing their effects on sludge properties and anaerobic digestion. Future challenges and concerns related to pre-treatment assessment and implementation are also addressed.     

Recovery of organic carbon and phosphorus from wastewater by Fe-enhanced primary sedimentation and sludge fermentation

A new chemically enhanced primary sedimentation (CEPS) and sludge fermentation process are developed for improved nutrient removal, energy saving and resource recovery in municipal wastewater treatment. The FeCl₃-based CEPS with a dosage of 20 mg-Fe/L can remove 75.6% of organic pollutants and 99.3% of PO₄-P on average from wastewater. Under natural fermentation conditions, the CEPS sludge undergoes effective hydrolysis and acidogenesis to produce volatile fatty acids (VFAs) and release phosphate as valuable resources. By using CEPS, around 27% of the organic carbon in wastewater influent can be recovered via sludge fermentation, mainly in the form of VFAs, and about 23% of phosphorus recovered for making vivianite fertilizer. In comparison, both the organic and phosphorus recovery ratios from wastewater are under 10% with conventional primary sedimentation and sludge fermentation. CEPS combined with side-stream sludge fermentation can be readily applied in new treatment plants or in a retrofit of existing treatment systems.     

Developments in odour control and waste gas treatment biotechnology: a review

Waste and wastewater treatment processes produce odours, which can cause a nuisance to adjacent populations and contribute significantly to atmospheric pollution. Sulphurous compounds are responsible for acid rain and mist; many organic compounds of industrial origin contribute to airborne public health concerns, as well as environmental problems. Waste gases from industry have traditionally been treated using physicochemical processes, such as scrubbing, adsorption, condensation, and oxidation, however, biological treatment of waste gases has gained support as an effective and economical option in the past few decades. One emergent technique for biological waste gas treatment is the use of existing activated sludge plants as bioscrubbers, thus treating the foul air generated by other process units of the wastewater treatment system on site, with no requirement for additional units or for interruption of wastewater treatment. Limited data are available regarding the performance of activated sludge diffusion of odorous air in spite of numerous positive reports from full-scale applications in North America. This review argues that the information available is insufficient for precise process design and optimization, and simultaneous activated sludge treatment of wastewater and airborne odours could be adopted worldwide.     

Enhancing mainstream nitrogen removal by employing nitrate/nitrite-dependent anaerobic methane oxidation processes

Due to serious eutrophication in water bodies, nitrogen removal has become a critical stage for wastewater treatment plants (WWTPs) over past decades. Conventional biological nitrogen removal processes are based on nitrification and denitrification (N/DN), and are suffering from several major drawbacks, including substantial aeration consumption, high fugitive greenhouse gas emissions, a requirement for external carbon sources, excessive sludge production and low energy recovery efficiency, and thus unable to satisfy the escalating public needs. Recently, the discovery of anaerobic ammonium oxidation (anammox) bacteria has promoted an update of conventional N/DN-based processes to autotrophic nitrogen removal. However, the application of anammox to treat domestic wastewater has been hindered mainly by unsatisfactory effluent quality with nitrogen removal efficiency below 80%. The discovery of nitrate/nitrite-dependent anaerobic methane oxidation (n-DAMO) during the last decade has provided new opportunities to remove this barrier and to achieve a robust system with high-level nitrogen removal from municipal wastewater, by utilizing methane as an alternative carbon source. In the present review, opportunities and challenges for nitrate/nitrite-dependent anaerobic methane oxidation are discussed. Particularly, the prospective technologies driven by the cooperation of anammox and n-DAMO microorganisms are put forward based on previous experimental and modeling studies. Finally, a novel WWTP system acting as an energy exporter is delineated.     

Assessment of physiological state of microorganisms in activated sludge with flow cytometry: application for monitoring sludge production minimization

Many sludge reduction processes have been studied for the minimization of sludge production in biological wastewater treatment. The investigations on most of these processes have monitored the increase of the soluble chemical oxygen demand, the sludge mass reduction, or the decrease of the floc size, but little information has been obtained on cell lysis and the change of the biological cell activity. However, employing any strategy for reducing sludge production may have an impact of microbial community in biological wastewater treatment process. This impact may influence the sludge characteristics and the quality of effluent. The objective of this study concerns the determination of the physiological state of activated sludge microorganisms during a sludge minimization process. A thermal treatment at 80 °C for 5, 20, 40 and 60 min was chosen in this study. Staining bacteria with CTC and SYTOX green was used to evaluate biological cell activity and viability of cell types contained in activated sludge, respectively. The monitoring of cell activity and viability was performed using flow cytometry (FCM) analysis before and after thermal treatment of activated sludge. Results indicated an increase in the number of permeabilized cells and a decrease in the number of active cells, subsequent to the thermal treatment. The study also confirms the potential of FCM to successfully evaluate the physiological heterogeneity of an activated sludge bacterial population. Moreover, the experimentally observed correlations between the FCM results and the organic matter solubilization in activated sludge samples during thermal treatment revealed that the increase in the soluble organic matter concentration was predominantly due to an intracellular material release. Identifying the increase in activated sludge hydrolysis requires a precise knowledge of the involved mechanisms, and this study indicated that the FCM, used in conjunction with specific probes, could be a useful tool.     

Sludge minimisation technologies

The treatment and disposal of excess sludge represents a bottleneck of wastewater treatment plants all over the world, due to environmental, economic, social and legal factors. There is therefore a growing interest in developing technologies to reduce the wastewater sludge generation. The goal of this paper is to present the state-of-the-art of current minimisation techniques for reducing sludge production in biological wastewater treatment processes. An overview of the main technologies is given considering three different strategies: The first option is to reduce the production of sludge by introducing in the wastewater treatment stage additional stages with a lower cellular yield coefficient compared to the one corresponding to the activated sludge process (lysis-cryptic growth, uncoupling and maintenance metabolism, predation on bacteria, anaerobic treatment). The second choice is to act on the sludge stage. As anaerobic digestion is the main process in sewage sludge treatment for reducing and stabilising the organic solids, two possibilities can be considered: introducing a pre-treatment process before the anaerobic reaction (physical, chemical or biological pre-treatments), or modifying the digestion configuration (two-stage and temperature-phased anaerobic digestion, anoxic gas flotation). And, finally, the last minimisation strategy is the removal of the sludge generated in the activated sludge plant (incineration, gasification, pyrolysis, wet air oxidation, supercritical water oxidation).     

Evaluation of methods to solubilize and analyze cell-associated ectoenzymes

A protocol for production, storage, and use of Shock 1 (Shk1) bioreporter cells for toxicity monitoring in wastewater treatment facilities was developed. Shk1 is a bioluminescent toxicity bioreporter for activated sludge previously constructed by the incorporation of lux genes into an activated sludge microorganism.

A number of factors affecting Shk1 growth and bioluminescence were examined including the growth medium, tetracycline concentration, storage conditions, and test media. Based on the results of these experiments, a toxicity testing protocol was developed that involved growth of cultures in nutrient broth with tetracycline, storage of cultures at 4 °C, cell activation by reinoculation into nutrient broth, and toxicity testing by cell injection into the test media. Effective use of this approach required standardized time intervals for cell growth, storage, activation and exposure in the test media.

Bioluminescence from Shk1 cells was measured in nutrient broth and influent wastewater and activated sludge mixed liquor from a municipal wastewater treatment plant. Using the Shk1 toxicity testing protocol, Zn EC₅₀ values for bioluminescence in nutrient broth, influent wastewater, and activated sludge mixed liquor were approximately 42, 7, and 32 mg/l, respectively. Zn concentrations as low as 1 mg/l could be detected in influent wastewater. The detection limit in influent wastewater is below the Zn concentrations typically reported to affect the activated sludge process.     

Carbon balance of anaerobic granulation process: carbon credit

The concept of carbon credit arose out of increasing awareness of the need to reduce emissions of greenhouse gases to combat global warming which was formalized in the Kyoto protocol. In addition to contribution to sustainable development with energy recovery in the form of methane, carbon credits can be claimed by application of advanced anaerobic processes in wastewater treatment for reducing emissions of greenhouse gases. As anaerobic granular systems are capable of handling high organic loadings concomitant with high strength wastewater and short hydraulic retention time, they could render much more carbon credits than other conventional anaerobic systems. This study investigated the potential carbon credit derived from laboratory-scale upflow anaerobic sludge blanket (UASB) reactors based on a carbon balance analysis. Methane emission reduction could be calculated by calculating the difference of UASB reactors and open lagoon treatment systems. Based on the 2.5l bench-scale reactor, the total CH(4) emissions reduction was calculated as 29 kg CO(2)/year. On scaling up to a typical full-scale anaerobic digester, the total CH(4) emissions reduction could achieve 46,420 tons CO(2) reduction/year. The estimated carbon credits would amount to 278,500 US$ per year by assuming a carbon price of 6 US$ per metric ton CO(2) reduction. The analysis postulated that it is financially viable to invest in advanced anaerobic granular treatment system from the revenue generated from carbon credits.